1. Field of the Invention
The present invention relates to a tuning system for a television signal received from a cable line or other source. In particular, this invention relates to a tuning system suitable for a television receiver provided with a scramble system which can tune a scrambled television signal.
2. Background of the Invention
Cable television (CATV) systems are very popular for providing television programs on a plurality of channels through a cable line provided within a certain geographic area. Such a CATV system includes a wideband transmission line which provides the television receiver at each home with several kinds of information and programs through a coaxial cable. Often such systems provide pay services such as databases and movies using a regular TV signal which undergoes a scrambling process. A specific signal descrambler (or decipher) is used to receive these pay services so that only subscribers who pay the cable companies and receive the descrambler can see the program.
In such a system, among the channels provided, a desired TV signal is selected by a tuning apparatus forming a part of the TV receiver. When the TV signal is scrambled, the above mentioned descrambler automatically converts the scrambled signal to a regular TV signal (video signal) to make it appear properly on the television screen.
A cable-ready TV receiver apparatus has been developed, which can be connected to the descrambler by an exclusive connecter (a so called multi-port connecter) so that cable pay programs can be obtained by merely purchasing the descrambler. Such a cable-ready TV receiver apparatus in general performs Automatic Frequency Tuning (AFT) operation when tuning. This keeps the receiver tuned to precisely receive the desired channel. A video signal demodulated in this condition is provided to the descrambler through the multi-port connecter. When the video signal is scrambled, a detectable information signal is inserted into the video signal to identify the scrambling process. The video signal, therefore, can be converted to regular (descrambled) video signal which is then sent back to the cable-ready TV receiver through the multi-port connecter. The proper image thus appears on the screen.
AFT has been widely adopted in cable-ready TV receivers for its tuning operation. As is well known, an AFT control signal can be obtained by detecting the Intermediate Frequency (IF) signal. This AFT signal is utilized as the variable voltage for a local oscillator circuit of the tuner to obtain precise tuning. The demodulated video signal is inspected to see if it contains a horizontal synchronizing signal or a vertical synchronizing signal. The presence of one of these signals confirms proper AFT operation.
To implement this confirmation, a counting operation is performed for either the horizontal synchronizing signal (hereafter H sync. signal) or vertical synchronizing signal (V sync. signal) over a certain time period. However, when a scrambled TV signal is received at a certain channel, H or V sync. signals are not present or corrupted by the scrambling process. The descrambling operation by the descrambler is therefore required for the above mentioned confirmation operation of the AFT. The time required for the descrambler to decipher the scrambling is generally 2-5 seconds depending on the scrambling method. As a result, when a scrambled program is selected at certain channel, it takes a relatively long time for the AFT operation to complete the tuning process. The user of the TV receiver apparatus is therefore inconvenienced.